Various transpiration and film cooling techniques have been used for reducing the deleterious effects of aerodynamic heating on high-speed aerospace vehicles. The primary emphasis has been the reduction in heating to prevent structural degradation of various surfaces on vehicles during operations such as reentry into the atmosphere or hypersonic atmospheric flight. Many prior art systems employ a gaseous coolant for simplicity in system design and control. These systems typically use a compressed inert gas such as nitrogen or helium stored on board the vehicle or air from the low-pressure compressor section of a jet engine powering the vehicle. The use of gaseous coolants does not take advantage of the latent heat of vaporization available through the use of phase change liquid or solid coolants. The limitation in cooling capacity of gaseous systems and the limits of storage capability for inert gas coolants makes the use of a phase change coolant desirable.
Liquid or solid phase change transpiration cooling systems have typically been designed in prior art systems to store the coolant at or near the surface which requires cooling. The coolant is vaporized by heat conduction from the surface to be cooled and the vapor is transpired through pores in the surface. Typically the pores in the surface have been sealed with a material which has been melted away by heating of the surface or is blown out by vapor pressure from the vaporized coolant. For use in the cooling of reentry vehicles and hypersonic flight vehicles, emphasis has been placed in the prior art on passive systems responding only to an increase of surface temperature beyond the vaporization point of the coolant. Sufficient coolant is therefore required for use during the entire period of potentially deleterious heating.
Typically, cooling systems for infrared signature reduction do not require continuous use. The temperatures of interest for reduction of infrared signature are usually well below the temperature for onset of any structural degradation. The use of a controllable system which is operable on command during critical mission segments when infrared signature reduction is required is therefore desirable. On board storage of the coolant may be remote from the surface to be cooled and the quantity of coolant may be based on an estimated time period for use independent of actual aerodynamic heating loads on the vehicle.
Prior art infrared signature reduction techniques included transmission of heat from the surface of a vehicle to interior structure or stored fuel through the use of fuel circulation or direct heat conduction techniques such as heat pipes. These techniques have only marginal utility due to the limited heat storage capability of such systems. In addition, the initial temperature gradient between the surface to be cooled and the structure or fuel storage decreases as heat is transported from the surface to the storage, thereby decreasing the heat transfer rate and reducing cooling capability.
The use of gaseous coolants in infrared signature reduction systems requires significant coolant flow rates due to the low heat capacity of gaseous coolants and low heat transfer coefficients characteristic of gas forced convection cooling. The concomitant plumbing size for satisfactory pressure losses to achieve the high flow rates is impractical for most surface cooling applications. A coolant system using a phase change to take advantage of the latent heal of vaporization offers approximately ten times the heat capacity of a system using only gas cooling. Therefore, a ten-fold savings in coolant weight may be obtained using a phase change system as compared to an open loop or sacrificial gaseous cooling system. Similarly, the weight of closed loop systems using only gaseous cooling and recirculating the coolant is prohibitive.
It is, therefore, desirable to have an efficient liquid-to-vapor phase change coolant system which is controllable to allow on command usage and accurate surface temperature control to provide an optimum infrared signature reduction system.